Infectious Disease Epidemiology

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Infectious Disease Epidemiology

• Principles of Epidemiology
• Lecture 7
• Dona Schneider, PhD, MPH, FACE

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Concepts in Infectious Epidemiology

• Agent, host, environment
• Classification of human infections by modes of
  transportation
• Incubation period
• Spectrum of disease
• Herd immunity

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Major Factors Contributing to the Emergence of
Infectious Diseases

• Human demographics and behavior
• Technology and industry
• Economic development and land use
• International travel and commerce
• Microbial adaptation and change
• Breakdown of public health measures

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Epidemiologic Triad Concepts

• Infectivity ability to invade a host
• ( infected / susceptible) X 100
• Pathogenicity ability to cause disease
• ( with clinical disease / of infected) X
  100
• Virulence ability to cause death
• ( of deaths / with disease (cases)) X 100
• All are dependent upon the condition of the host
• Immunity (active, passive)
• Nutrition
• Sleep
• Hygiene

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Mode of Transmission

• Person-to-person (respiratory, orogenital, skin)
• Examples HIV, measles
• Vector (animals, insects)
• Examples rabies, yellow fever
• Common vehicle (food, water)
• Examples salmonellosis
• Mechanical vectors (personal effects) such as
  doorknobs, or toothbrushes are called FOMITES

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Classification by Mode of Transmission

• Dynamics of Spread through Human Populations
• Spread by a common vehicle
• Ingestion Salmonellosis
• Inhalation Legionellosis
• Inoculation Hepatitis
• Propagation by serial transfer from host to host
• Respiratory Measles
• Anal-oral Shigellosis
• Genital Syphilis

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• Principle Reservoir of Infection
• Man Infectious hepatitis
• Other vertebrates (zoonoses) Tularemia
• Agent free-living Histoplasmosis
• Portal of Entry/Exit in Human Host
• Upper respiratory tract Diphtheria
• Lower respiratory tract Tuberculosis
• Gastrointestinal tract Typhoid fever
• Genitourinary tract Gonorrhea
• Conjunctiva Trachoma
• Percutaneous Leptospirosis
• Percutaneous (bite of arthropod) Yellow fever

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• Cycles of Infectious Agent in Nature
• Man-man Influenza
• Man-arthropod-man Malaria
• Vertebrate-vertebrate-man Psittacosis
• Vertebrate-arthropod-vertebrate-man Viral
  encephalitis
• Complex Cycles
• Helminth infections River blindness

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Incubation Period

• The interval between the time of contact and/or
  entry of the agent and onset of illness (latency
  period)
• The time required for the multiplication of
  microorganisms within the host up to a threshold
  where the parasitic population is large enough to
  produce symptoms

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• Each infectious disease has a characteristic
  incubation period, dependent upon the rate of
  growth of the organism in the host and
• Dosage of the infectious agent
• Portal of entry
• Immune response of the host
• Because of the interplay of these factors,
  incubation period will vary among individuals
• For groups of cases, the distribution will be a
  curve with cases with longer incubation periods
  creating a right skew

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Spectrum of Disease

• Exposure
• Subclinical manifestations
• Pathological changes
• Symptoms
• Clinical illness
• Time of diagnosis
• Death
• Whether a person passes through all these stages
  will depend upon infection and prevention,
  detection and therapeutic measures

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Iceberg Concept of Infection
CELL RESPONSE
HOST RESPONSE
Lysis of cell
Fatal
Discernable effect
Clinical and severe disease
Clinical Disease
Cell transformation or Cell dysfunction
Moderate severity Mild Illness
Incomplete viral maturation
Infection without clinical illness
Below visual change
Subclinical Disease
Exposure without cell entry
Exposure without infection
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Spectrum of Disease (cont.)

• Example
• 90 of measles cases exhibit clinical symptoms
• 66 of mumps cases exhibit clinical symptoms
• lt10 of poliomyelitis cases exhibit clinical
  symptoms

Inapparent infections play a role in
transmission. These are distinguished from
latent infections where the agent is not shed
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Subclinical/Clinical Ratio for Viral Infections
Virus
Age at infection
Clinical feature
Clinical cases
Estimated ratio
10001
Child
Paralysis
0.1 to 1.0
Polio
1 to 5 years
Mononucleosis
1
gt 1001
Epstein-Barr
1 to 10
101 to 1001
6 to 15 years
21 to 31
16 to 25 years
50 to 75
5
201
lt 5 years
Icterus
Hepatitis A
10
111
5 to 9 years
10 to 15 years
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71
80 to 95
1.51
Adult
50
21
5 to 20 years
Rash
Rubella
60
1.51
Young adult
Fever, cough
Influenza
gt99
199
5 to 20 years
Rash, fever
Measles
lt110,000
CNS symptoms
gtgtgtgt99
Any age
Rabies
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Herd Immunity

• The decreased probability that a group will
  develop an epidemic because the proportion of
  immune individuals reduces the chance of contact
  between infected and susceptible persons
• The entire population does not have to be
  immunized to prevent the occurrence of an
  epidemic
• Example smallpox, measles

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Investigating an Epidemic

• Determine whether there is an outbreak an
  excess number of cases from what would be
  expected
• There must be clarity in case definition and
  diagnostic verification for each case

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Investigating an Epidemic (cont.)

• Plot an epidemic curve (cases against time)
• Calculate attack rates
• If there is no obvious commonality for the
  outbreak, calculate attack rates based on
  demographic variables (hepatitis in a community)
• If there is an obvious commonality for the
  outbreak, calculate attack rates based on
  exposure status (a church supper)

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Investigating an Epidemic (cont.)

• Determine the source of the epidemic
• If there is no obvious commonality for the
  outbreak, plot the geographic distribution of
  cases by residence/work/school/location to reduce
  common exposures
• If there is an obvious commonality for the
  outbreak, identify the most likely cause and
  investigate the source to prevent future outbreaks

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• Index Case
• Person that comes to the attention of public
  health authorities
• Primary Case
• Person who acquires the disease from an exposure
• Attack rate
• Secondary Case
• Person who acquires the disease from an exposure
  to the primary case
• Secondary attack rate

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Calculation of Attack Rate for Food X
Did not eat the food (not exposed)
Ate the food (exposed)
Attack Rate
Attack Rate
Total
Well
Total
Well
Ill
Ill
64
11
4
7
76
13
3
10
Attack Rate Ill / (Ill Well) x 100 during
a time period
Attack rate (10/13) x 100 76
( 7/11) x 100 64 RR 75/64 1.2
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Secondary Attack Rate
Secondary attack rate ()
Total number of cases initial case(s)

x 100
Number of susceptible persons in the group
initial case(s)

• Used to estimate to the spread of disease in a
  family, household, dorm or other group
  environment.
• Measures the infectivity of the agent and the
  effects of prophylactic agents (e.g. vaccine)

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Mumps experience of 390 families exposed to a
primary case within the family
Cases
Population
No. susceptible before primary cases occurred
Secondary
Primary
Total
Age in years
50
100
250
300
2-4
87
204
450
5-9
420
15
25
84
152
10-19
Secondary attack rate 2-4 years old
(150-100)/(250-100) x 100 33
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Case Fatality Rate
Number of deaths due to disease X
Case fatality rate ()

x 100
Number of cases of disease X

• Reflects the fatal outcome (deadliness) of a
  disease, which is affected by efficacy of
  treatment

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• Assume a population of 1000 people. In one year,
  
• 20 are sick with cholera and 6 die from the
  disease.
• The cause-specific mortality rate in that year
  from cholera
• The case-fatality rate from cholera

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0.006

0.6
1000
6

0.3

30
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